Is there an upper threshold of pCO₂ that can cause symptomatic hypercapnia (e.g. AMS) despite metabolic compensation and normal pH?  

Hi - I'm not sure about an upper threshold of pCO2 and AMS. However, even with normal pH, elevated pCO2 can cause significant increases in cerebral blood flow. Pollock et al., PMID 19406361 studied MR perfusion imaging and found that patients with a mean pCO2 of ~ 54mmHg had more than double the cer...